A federal judge has slashed a damages award Bayer AG owed a California man who blamed Roundup weed killer for his cancer, to $US25.27 million ($A35.90 million) from $US80.27 million, while rejecting the company's bid for a new trial.

Evidence against the former Monsanto Co, which Bayer bought last year, supported the $US5.27 million in compensatory damages that a jury awarded Edwin Hardeman, US District Judge Vince Chhabria in San Francisco said.

He also said the jury acted reasonably in awarding punitive damages.

Chhabria nonetheless reduced punitive damages to $US20 million from $US75 million, saying that while Monsanto "deserves to be punished", the higher award was "constitutionally impermissible" because it was nearly 15 times the compensatory damages award.

"Monsanto's conduct, while reprehensible, does not warrant a ratio of that magnitude, particularly in the absence of evidence showing intentional concealment of a known or obvious safety risk", Chhabria wrote.

Hardeman said he used Roundup for many years starting in the 1980s to treat poison oak and weeds on his property. He was diagnosed with non-Hodgkin's lymphoma in 2014 but is now in remission.

Hardeman is one of more than 13,400 plaintiffs who have sued Bayer and Monsanto over Roundup, saying the herbicide's active ingredient, glyphosate, is unsafe.

In a statement, Bayer called Chhabria's decision "a step in the right direction," but said it still planned to appeal.